I feel like it depends on the age. If the MC's a student, then yeah, it's fine that they're clueless about some stuff. But if they're 25+ and still acting like a child?? Bro... that's just cringe
I believe it's contingent upon the plot. On the assumption that the mc grew up being mistreated, it is totally inevitable.. in addition to that I don't think we can blame her in the case of suffering multitudinous trauma.
There are both good and bad representations of having a naive Fl. In my opinion, I don't really like gullible Fls. I find that they are way too prevalent in media these days.
A good instance could be if eventually she learns and grows from her naivety. Taking her mistakes and using them to better herself. Usually though, that is never the case. In the end, the Fl will continue to cling on to her LI. Forever cast in the shadows of those who continue to better themselves.